CIS110 Computer Programming Design Chapter (3)

Programming Logic and DesignEighth Edition

Chapter 3Understanding Structure

Objectives

In this chapter, you will learn about:• The disadvantages of unstructured spaghetti code• The three basic structures—sequence, selection, and

loop• Using a priming input to structure a program• The need for structure• Recognizing structure• Structuring and modularizing unstructured logic

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The Disadvantages of Unstructured

Spaghetti Code• Spaghetti code– Logically snarled program statements– Often a complicated mess– Programs often work but are difficult to read and maintain– Confusing and prone to error

• Unstructured programs– Do not follow the rules of structured logic

• Structured programs – Follow the rules of structured logic

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4Programming Logic and Design, Eighth Edition Figure 3-1 Spaghetti code logic for washing a dog

Understanding the Three Basic Structures

• Structure– Basic unit of programming logic – Sequence structure

• Perform actions or tasks in order• No branching or skipping any task

– Selection structure (decision structure)• Ask a question, take one of two actions• Often called if-then-else• Dual-alternative ifs or single-alternative ifs

– Loop structure• Repeat actions while a condition remains true

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Understanding the Three Basic Structures (continued)

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Figure 3-2 Sequence structure

Understanding the Three Basic Structures (continued)

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Figure 3-3 Selection structure

Understanding the Three Basic Structures (continued)

• Dual-alternative ifs– Contains two alternatives– The if-then-else structure

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if someCondition is true thendo oneProcess

elsedo theOtherProcess

endif

Understanding the Three Basic Structures (continued)

• Single-alternative ifs

– An else clause is not required• null case– Situation where nothing is done

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if employee belongs to dentalPlan thendeduct $40 from employeeGrossPay

Understanding the Three Basic Structures (continued)

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Figure 3-4 Single-alternative selection structure

Understanding the Three Basic Structures (continued)

• Loop structure– Repeats a set of actions while a condition remains true

• Loop body– Also called repetition or iteration– Condition is tested first in the most common form of loop– The while…do or while loop

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Understanding the Three Basic Structures (continued)

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Figure 3-5 Loop structure

Understanding the Three Basic Structures (continued)

• Loop structure

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while testCondition continues to be truedo someProcess

endwhilewhile you continue to be hungry

take another bite of food

determine if you still feel hungry

endwhile

Understanding the Three Basic Structures (continued)

• All logic problems can be solved using only sequence, selection, and loop

• Structures can be combined in an infinite number of ways

• Stacking structures– Attaching structures end-to-end

• End-structure statement– Indicates the end of a structure– The endif statement ends an if-then-else structure– The endwhile statement ends a loop structure

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Understanding the Three Basic Structures (continued)

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Figure 3-6 Structured flowchart and pseudocode with three stacked structures

Understanding the Three Basic Structures (continued)

• Any individual task or step in a structure can be replaced by a structure

• Nesting structures– Placing one structure within another– Indent the nested structure’s statements

• Block– A group of statements that execute as a single unit

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Understanding the Three Basic Structures (continued)

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Figure 3-7 Flowchart and pseudocode showing nested structures—a sequence nested within a selection

Understanding the Three Basic Structures (continued)

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Figure 3-8 Flowchart and pseudocode showing nested structures—a loop nested within a sequence, nested within a selection

Understanding the Three Basic Structures (continued)

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Figure 3-9 Flowchart and pseudocode for a selection within a loop within a sequence within a selection

Understanding the Three Basic Structures (continued)

• Structured programs have the following characteristics:– Include only combinations of the three basic structures– Each structure has a single entry point and a single exit

point– Structures can be stacked or connected to one another

only at their entry or exit points– Any structure can be nested within another structure

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Using a Priming Input to Structurea Program

• Priming input (or priming read)– Reads the first input data record– Is outside the loop that reads the rest of the records– Helps keep the program structured

• Analyze a flowchart for structure one step at a time• Watch for unstructured loops that do not follow this

order– First ask a question– Take action based on the answer– Return to ask the question again

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Using a Priming Input to Structurea Program (continued)

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Figure 3-14 Structured, but nonfunctional, flowchart of number-doubling problem

Using a Priming Input to Structurea Program (continued)

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Figure 3-15 Functional but unstructured flowchart

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Figure 3-16 Functional, structured flowchart for the number-doubling problem

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Figure 3-17 Structured but incorrect solution to the number-doubling problem

Understanding the Reasons for Structure

• Clarity—unstructured programs are confusing• Professionalism—other programmers expect it• Efficiency—most languages support it• Maintenance —other programmers find it easier to

read• Modularity —easily broken down into modules

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Figure 3-18 Example 1

Structured Flowcharts?

Recognizing Structure

Figure 3-19 Example 2

Recognizing Structure (continued)

28Programming Logic and Design, Eighth EditionFigure 3-20 Example 3

An Unstructured Flowchart

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Figure 3-21 Steps to structure the dog-washing process

Recognizing Structure (continued)

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Figure 3-22 Structured dog-washing flowchart and pseudocode

Recognizing Structure (continued)

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Figure 3-23 Modularized version of the dog-washing program

Recognizing Structure (continued)

Summary

• Spaghetti code– Statements that do not follow rules of structured logic

• Three basic structures– Sequence, selection, and loop– Combined by stacking and nesting

• Priming input– Statement that reads the first input value prior to starting

a structured loop

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Summary (continued)

• Structured techniques promote: – Clarity– Professionalism– Efficiency– Modularity

• Flowcharts can be made structured by untangling• Logical steps can be rewritten to conform to the

three structures

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